Ring networks have been put into practical use as one form of a network configuration. In a ring network, a transmission line cable (such as an optical fiber cable) may be efficiently wired. Moreover, systems for transmitting Ethernet (registered trademark) packets by using a ring network have been popularized.
In a ring network using Ethernet transmission technology, a transmission path of packets may become a loop. Therefore, a ring network has the problem of a so-called loop phenomenon (a state where packets continue to go around in a network, causing the volume of network traffic to be continuously heavy).
This problem is solved, for example, by a configuration for providing a blocking port. In examples illustrated in FIG. 1 and FIG. 2, a ring network includes communication nodes #a-#f, and communication node #a is defined as a master node. The master node provides a blocking port in order to prevent a loop from occurring. In the example illustrated in FIG. 1A, communication node #a terminates a packet between communication nodes #a and #b.
Additionally, it is preferable that a communication network has a function for automatically recovering from a fault. Also, for a ring network, a recovery function is proposed. Assume that a fault has occurred between communication nodes #d and #e as illustrated in FIG. 1A. In this case, communication nodes #d and #e respectively transmit a fault notification to the master node (here, communication node #a). Then, communication node #a releases the blocking port. This allows a packet to be transmitted between communication nodes #a and #b, and each of the communication nodes is able to transmit a packet to a desired communication node. At this time, communication nodes #d and #e may provide a blocking port.
When the fault between communication nodes #d and #e is recovered, communication nodes #d and #e respectively transmit a recovery notification to the master node as illustrated in FIG. 1B. Then, communication node #a provides the blocking port. Moreover, communication nodes #d and #e respectively release the blocking port according to an instruction issued from the master node. This procedure for recovering from a fault is proposed, for example, by ITU-T Recommendation G.8032.
Furthermore, a method for providing a connection communication by using the above described ring network has been proposed in recent years. For example, as illustrated in FIG. 2, a path that passes through communication nodes #f, #e, #d, and #c is established when data is transmitted from a transmitting station 101 to a receiving station 102.
As a related technique, a packet relay device that reduces a communication disconnection time by avoiding congestion occurring when a fault occurs or when a fault is recovered in a ring packet network is proposed (for example, Japanese Laid-open Patent Publication No. 2006-270169).
As another related technique, a configuration for protecting a frame that passes through an inter-ring bridge node when a fault occurs in the bridge node in a system including a plurality of ring networks is proposed (for example, Japanese Laid-open Patent Publication No. 2003-229876).
The conventional techniques have a problem in operations performed when a path that secures connectivity is set in a ring network, and a suitable or efficient redundant configuration is not implemented. Namely, in the conventional ring network illustrated in FIG. 1A and FIG. 1B, it was difficult to perform operations by applying an alternate path in advance because of the existence of a blocking port. In other words, the conventional techniques have a problem of being unable to sufficiently provide a redundant configuration.